In MIMO wireless communication systems, multiple spatial streams are simultaneously transmitted via multiple antennas of a first wireless device to a second wireless device. When applying beamforming weights to the signals that form the multiple spatial streams to be transmitted, knowledge of the channel information with respect to all of the antennas of the second wireless device is needed for accurate computation of the beamforming weights. In some cases, this channel information is readily available when the second wireless device transmits the same number of spatial streams to the first wireless device. This is not the case, however, when the second wireless device transmits a lesser number of spatial streams to the first wireless device.